Why electric utilities care about fault currents

In many networks, the electrical currents — called “fault currents” — that arise , when short-circuits (“faults”) suddenly appear *, are expected to overwhelm the networks’ circuit breakers (whose sole purpose is to isolate those faults from the rest of the network) unless something is done to avert this possibility or reduce its likelihood to an acceptable level.This matter is important to utilities, their regulators and the general public because The result of rampaging fault currents can be the speedy destruction of some equipment, followed in the near or far future by fires in other equipment — the latter made possible by damage to that equipment’s electrical insulation by past fault currents.

Because of their extraordinary magnitude, fault currents exert extraordinary forces on each other. These can literally tear apart transformers, bus work and overhead powerlines.

Because of their extraordinary magnitude, fault currents raise the temperature of conductors and thereby damage their electrical insulation. Fire may be a subsequent consequence.

* Short circuits can be caused by: (1) physical damage (e.g. shifting of underground structures, inadvertently digging up a cable, underground steam heating or impingement), moisture and/or contaminant intrusion (e.g. salt/snow melting into damaged insulation), (2) aging mechanisms (thermal mechanical bending,treeing of insulation), (3) the stress of another nearby fault and/or (4) a combination of any of these mechanisms over time. Once a fault occurs, the fault current takes the easiest (lowest impedance) path to ground (earth), and all power sources that are electrically connected to the fault send power to it. The closer they are, the more these power sources contribute to the fault current. Currents from the separate power sources concentrate near the fault, resulting in a total fault current that can be hundreds of times larger than usual. In and of themselves, these currents can produce very high mechanical, magnetic and thermal stresses that may age, damage or weaken equipment and so make additional faults more likely in the future.

And what they want

The utilities want a device that will limit fault currents to a magnitude that their already installed circuit breakers, transformers , insulators and supports can handle. The desired device should have a negligible impact on the network when it is not needed, but it must be able to spring into action almost instantly (i.e. before the first peak to avoid weakening or destruction of mechanical bracing for equipment and supports)  and repeatedly, when necessary, because several faults can occur in rapid succession, or the utility may (re)close its circuit breakers while one fault persists and so have to again limit the fault current. (Indeed, some utilities open and close their breakers three times before concluding that a persistent fault is causing the problem.) These requirements rule out using a fuse, which can isolate a fault but cannot be used a second time. Of course, the desired device also should be small, reliable, inexpensive to maintain, and less expensive than refurbishing the whole network or replacing the equipment that the fault current limiter is intended to protect.

Answers to some related questions

The essence of the issue is presented above, but a few remarks here may anticipate some questions.

First, why not “upgrade” the circuit breakers? In many cases that would very, very expensive, and in some cases, the already installed breakers are the biggest ones available.

Second, how did this potential for overwhelming fault currents arise? Most often, it has come about from adding new generation to an existing transmission and distribution network; in other cases, it is a consequence of adding new interconnections, which enable more existing generators to pump power into the same fault. Of course, during normal operation both more generation and more interconnection are desired. The first enables the utility to satisfy increasing end-user demands for power, and the second gives the utility more flexibility in the choice of generating stations.

Third, can we expect future fault currents to be even larger than those of present concern? In many places, the answer is yes. The magnitude of the fault current is determined by the impedance between the generators and the fault. Plans to distribute generation throughout the network, bringing generation closer to end-users than is now usual, would tend to reduce the impedance that limits fault currents. For example, plans to locate renewable generation near end-users would entail decreased impedance between generator and fault, and thus, increased fault-currents. In general, fault current limiters now being developed would make renewable generation more attractive.

Fourth, just how and why do faults suddenly appear? From time to time, all of the following cause faults: lightning strikes, wind-storms, ice-storms, tree branches touching power lines, animals touch two conductors simultaneously (e.g. leaping squirrels, even snakes -they can work there way into equipment), and animals gnaw away electrical insulation. In addition, people sometimes disrupt service, as shown below.

Though undesired, short circuits or “faults”, can suddenly appear for many reasons.